1. The ultraviolet spectrum of solar light
The sun is a light source which has the entire ultraviolet spectrum as is shown in TABLE I. The portion of the solar spectrum which has carcinogenic properties covers the wavelength range from 290 to 320 nm. This portion of the ultraviolet spectrum is designated UVB, differentiating it from UVA, which ranges from 320 to 400 nm, and UVC, which ranges from 200 to 290 nm. Only UVA and UVB portions of the ultraviolet spectrum can reach the earth; the UVC portion, the most dangerous radiation to humans, is filtered out by the ozone layer surrounding the earth. The ozone layer also absorbs a part of UVB, thus reducing the prevalence of skin cancers in humans. Unfortunately, the damage caused by man to the ozone layer impairs its protective function, which contributes to the rapid increase in the incidence of skin cancer in humans
TABLE I ______________________________________ THE ULTRAVIOLET SPECTRUM OF SOLAR LIGHT UV radiation band Description Spectrum Comments ______________________________________ UVA Long-wave, near Short-wave Not absorbed by UV black light 320-340 nm ozone layer. Long-wave Short-wave UVA 340-400 nm may be harmful to humans. UVB Middle UV, "sun- 290-320 nm A portion is burn" radiation absorbed in the ozone layer. UVC Short-wave, far 200-290 nm Absorbed by UV germicidal ozone layer in radiation atmosphere ______________________________________
Exposure to solar radiation causes vasodilation and an increase in the volume of blood in the dermis, which is perceived visually as a reddening of the skin. Effects of solar radiation to human skin include tanning, photoaging and photocarcinogenesis.
2. The tanning effect
There are two different types of tanning resulting from exposure to solar radiation. They are immediate tanning and delayed tanning. Immediate tanning or immediate pigment darkening is a transient grayish brown discoloration of the skin induced by exposure to UVA and other visible light of certain wavelengths. It begins during the exposure and is maximal at the end of the radiation period. Persistence of the effect depends on the duration of exposure. Immediate pigment darkening will begin fading within minutes following a short exposure but will last more than 36 hours after prolonged exposure to high-intensity UVA. The immediate pigment darkening has been believed to give ultraviolet protective action for a long time, although there were claims that question the effectiveness of this action. The degree of the immediate darkening is dependent on the skin type of an individual, the extent of previous tanning, and the exposure dose, the darker the nature of the skin and previously acquired tan, the more pronounced the response.
Delayed tanning occurs 48 to 72 hours following the ultraviolet exposure, peaks in 7 to 10 days, and persists for several weeks to months. It can be maintained by repeated exposure to ultraviolet radiation. Delayed tanning involves much more complex reactions in skin. Both UVA and UVB induce delayed tanning. The latter is much more effective in inducing tanning than the former.
The same degree of skin tanning can be achieved differently. Tanning can follow sunburning with UVB or can occur over a long period when an individual is subjected to multiple low exposure. Obviously, the latter is a more healthful way to obtain skin tanning.
Because of the delayed appearance, the tanning degree of the delayed tanning cannot be evaluated at the time of sunbathing and at a real time basis during the sunbathing. Therefore, there is a large possibility that an individual will receive a higher dose of solar ultraviolet radiation than is required to achieve a desired tanning degree. This may result in not only more tanning, but may also subject the individual to the dangers of skin cancer induced by excessive exposure to solar ultraviolet radiation.
Therefore, a device is needed to alert an individual with information about the daily accumulated dose of solar ultraviolet radiation predetermined to provide a desired skin tanning degree over a specified number of days.
3. The carcinogenesis properties of solar ultraviolet radiation
Skin cancers are the most common cancers in human populations, with more than 550,000 new cases reported annually in the United States. This is responsible for at least 30% of the annual primary cancer diagnoses in the country. The occurrence rate of skin cancer has been increasing dramatically each year.
It has been well recognized that solar ultraviolet radiation is the predominant causal factor in cutaneous carcinogenesis in humans. Experimental studies indicated that the ultraviolet B (UVB; 290-320 nm) portion of the solar spectrum is primarily responsible for the carcinogenesis properties of sunlight. Solar ultraviolet radiation is responsible for 90 percent of squamous and basal cell carcinomas (SCC and BCC). It has also been implicated in melanoma, which accounts for only 7 percent of skin cancers, but for 80 percent of deaths from skin cancer. These three types of cancers account for nearly all the cases of skin cancer.
Epidemiologic studies revealed important characteristics of skin cancer in humans. The prevalence of human skin cancer correlates inversely with the degree of melanogenic pigmentation and with the geographic latitude away from the equator. A dramatic increase in the prevalence of skin cancer in light-complexioned Celts who migrated from the British Isles to Australia has also been reported. More than 90% of skin cancer in white persons occurs on sun-exposed areas of the body; and such lesions occur more frequently in individuals who receive greater sun exposure. It can be concluded that the prevalence of skin cancers is considerably higher in the sunbelt states, in the white population, and in locations near the Equator. The longterm clinical and epidemiologic studies provided strong evidence to support this conclusion.
About one hundred years ago, attention was focused on the clinical degenerative and neoplastic changes seen in the sun-exposed skin of sailors. In 1920, observations were reported that grape pickers in the vineyards of Bordeaux, France developed cancers commonly in sun-exposed skin but only rarely in protected skin. In 1928, experiments were conducted to demonstrate irritation from a mercury arc light source to induce skin cancers in laboratory animals. It was also found that short wave light (wavelength less than 320 nm) is responsible for carcinogenesis in mice. This finding was further supported by numerous studies which confirmed that the light of wavelengths ranging from 290-320 nm is a highly effective inducer and promoter of skin cancer in rodents.
Over the years, extensive efforts have been directed to discovering the mechanisms of the UV induction of skin cancers. Results of in vivo experiments using laboratory animals have clearly demonstrated that skin cancer is associated with a failure of the immune system to recognize tumor antigens and/or to destroy malignant cells. Recent observations concern aberrations in the appearance and distribution of Langerhans' cells (LCs) in the epidermic components of squamous cell carcinomas and suggest that epidermic LCs, which are considered the outmost sentinels of the immune system, play a relevant role in the development of UV-induced skin cancers in humans. Other studies further suggested that UV radiation not only induces skin cancers in laboratory animals but also promotes their development and causes the emergence of tumor-specific "suppressor" T cells (Ts) that prevent the immunologic destruction of these cancers.
In conclusion, solar UV radiation can initiate and promote skin carcinogenesis in humans. UVB damages DNA and suppresses the functions of the immune system defending the intruders, which allow tumors to grow.
4. Factors affecting the risk degree of solar UV radiation The risk degree of solar ultraviolet radiation is individually different. Many factors may contribute to this difference. They include:
an individual's skin type and its condition PA0 spectrum of the ultraviolet radiation PA0 intensity of the ultraviolet radiation PA0 total exposure time PA0 time of day of exposure
1) Individual's skin type and its condition
Human skin may be divided into four types according to their risk degree to be damaged from solar ultraviolet exposure. Statistics obtained from a study involving 104 patients show the following percentages distribution by skin type:
TABLE II ______________________________________ SKIN TYPE Skin Type % Comments ______________________________________ I 6 Always burns easily, never tans II 27 Burns moderately, tans gradually and uniformly III 63 Burns minimally, always tans well IV 5 Rarely burns, tans profusely ______________________________________
The minimal UV radiation dose initiating the skin damage (erythemal) also depends on the condition of the skin, including its thickness, amount of melanin in the skin, and its moisture. People should be knowledgeable about their skin type and conditions. Although one cannot easily change them, at least this knowledge helps one to learn his risk degree to solar ultraviolet radiation.
2) Spectrum of ultraviolet radiation
The damage degree of ultraviolet radiation to skin is dependent upon the spectrum of the UV radiation. UVB radiation, particularly the shorter wavelengths, is most efficient in causing erythemal and sunburn. All UVA radiation can also cause erythema, but levels 800 to 1000 times higher than those of UVB are required. That suggests the risk degree of UVA is a lot smaller than that of the same dose of UVB, but it is not 800 to 1000 times smaller. The reason is that there is much more UVA in the solar spectrum than UVB.
As the wavelength increases, the risk factor decreases. At wavelengths above 340 nm (long UVA), erythema may be difficult to detect except in highly susceptible people, such as those with type I skin.
The spectrum of ultraviolet radiation is variable during the day and according to the weather conditions. The following lists the intensity ratio of the UVB to UVA at different times during the day.
TABLE III ______________________________________ UV RADIATION vs. DAY TIME UV RADIATION RATIO TIME INTENSITY (UVB:UVA) ______________________________________ &lt;10:00 am Moderate &lt;1:1 10:00 am-2:00 pm Strongest especially 7+:1 in summer &gt;2:00 pm Moderate &lt;1:1 ______________________________________
Weather conditions affect the effect of solar ultraviolet radiation. Higher humidity will increase UV penetration, especially at shorter wavelengths. Some people believe that clouds can attenuate UV radiation. Clouds can absorb only infrared radiation, but not ultraviolet radiation. Therefore, solar ultraviolet damage occurs at nearly the same rate on hazy days.
3) Intensity of the ultraviolet radiation
It is easy to understand that over the same period of time of exposure to solar ultraviolet radiation, the higher the intensity of the UV radiation, the higher the risk of skin damage.
4) Total exposure time.
Same as 3).
5) Time of day of exposure
Because the intensity of solar ultraviolet radiation and the ratio of the UVB to UVA is variable throughout the day, sunbathing between 10:00 am and 2:00 pm, especially in summer, increases the risk of UV damage to skin over sunbathing before 10:00 am or after 2:00 pm.
In view of the importance of preventing undesired damage to skin caused by overexposure of skin to solar ultraviolet radiation and the desire of man for skin tanning, it is desired to provide improved devices for measuring levels and cumulative amounts of ultraviolet exposure and to apportion a tanning amount of ultraviolet radiation on a daily basis over a period of days to obtain a desired tanning. It is also desired to provide an improved method for safe and controlled tanning.